US20010026733A1

US20010026733A1 - Low heave motion semi-submersible vessel

Info

Publication number: US20010026733A1
Application number: US09/789,688
Authority: US
Inventors: Robert Ludwigson
Original assignee: GVA Consultants AB
Current assignee: GVA Consultants AB
Priority date: 2000-02-23
Filing date: 2001-02-21
Publication date: 2001-10-04
Also published as: SG165142A1; NO20010871L; KR20010085480A; BR0100653A; NO20010871D0

Abstract

A semi-submersible vessel 10 adapted to resist heave motion that includes a plurality of support columns 16, each of which has an upper end 19 and a lower end 23. A deck 39 is interconnected between the plurality of support columns 16 at locations upon the columns 16 proximate the upper ends 19 thereof. The deck 39 is a box-shaped structure. A ring-pontoon 43 is connected to the plurality of support columns 16 at locations proximate the lower ends 23 of each of the columns 16. Each support column 16 has a longitudinal axis 29 that is inclined upwardly and inwardly from the ring-pontoon 43 to the deck 39 and the longitudinal axis is oriented substantially radially from a center point 13 of the vessel 10. An angle of columnar inclination of the support columns 16 approaches at least five degrees from vertical. The ring-pontoon 43 is rectangular-shaped, and in one variation, is substantially square-shaped and has corner pontoon portions 49 interconnected by elongate pontoon portions 56. The square-shaped ring-pontoon has a centerline 73 intersecting center points 79 of vertically taken cross-sections 76 of the square-shaped pontoon 43. The elongate pontoon portions 56 comprise four box-shaped structures 59, each of which has a substantially four-sided vertically taken cross-sectional configuration.

Description

RELATED PATENT APPLICATIONS

This patent application claims priority to U.S. Provisional Application No. 60/184,463 filed Feb. 23, 2000. Said application in its entirety is hereby expressly incorporated by reference into the present application.[0001]

TECHNICAL FIELD

The present invention relates generally to off-shore semi-submersible platforms, and more specifically to semi-submersible platforms where low heave motion is required.

BACKGROUND ART

Offshore platforms of the so called semi-submersible type typically include a superstructure deck carried upon columns supported by submersible pontoon system. The pontoon system can include either two or more elongate and usually parallel pontoon bodies, each of which normally extend beyond the columns. An alternative configuration for the submersible portion of the structure is a ring-pontoon construction. The ring-pontoon type of construction typically consists of four box-shaped structures formed together into a square ring.

While semi-submersibles with parallel pontoons normally have a number of braces between the pontoons and/or columns for taking up structural loads caused by wave-forces acting on the pontoons, ring-pontoon type semi-submersibles have the advantage of not requiring such braces. In the case of the ring-pontoon type semi-submersible, the structure of the pontoon ring itself forms a sufficiently strong and rigid structural member that interconnects the columns. As a result, no braces are needed. On the downside, however, the traditional ring-pontoon design for semi-submersibles has resulted in a pontoon geometry that detrimentally produces higher heave motions than counterpart semi-submersibles of similar capacities that have parallel pontoons. Such a traditional ring-pontoon configuration may be appreciated in FIG. 4 where a portion of one is superimposed on the left-hand side thereof. Still further, the traditional ring-pontoon configuration suffers from higher resistance characteristics when moved through the water, give that at least two sides of the ring-pontoon are oriented transversely to the direction of travel thereby creating significant drag when the semi-submersible is being moved between locations.

Ring-pontoon styled semi-submersibles have normally been used for the production of hydrocarbons. In this connection, the semi-submersible is held stationary at a location for long periods of time. These periods may extend over a number of years, and as a result the resistance experienced when the semi-submersible is moved is of negligible importance. On the other hand, as a result of the strong and rigid structural characteristics provided by the ring-pontoon, stresses induced in the platform are minimized and accordingly, so is the risk of cracks being caused in the platform.

When used in the production of hydrocarbons, semi-submersible platforms have normally been connected to sub-sea wellheads with flexible hose-type riser(s). Recently, however, there have been two developments in this area. First, there has been an increased interest or demand for so-called steel catenary risers. In this configuration, a substantially rigid steel, or other metal, pipe is configured so that a portion of the pipe lies along the sea floor and another portion extends upwards toward the water's surface in a catenary curve to a semi-submersible vessel. Due to the rigidity of the pipe, large heave motions (substantially vertical movements caused in a platform) of the semi-submersible result in considerable fatigue loading on such steel catenary risers. For ring-pontoon styled semi-submersibles, the larger heave-motions experienced have meant that steel catenary riser configurations have been less feasible to use in combination with such ring-pontoon designs in comparison to semi-submersibles having parallel pontoons.

In some circumstances, it is preferred to have dry wellheads located above the sea surface. These dry wellheads are connected to the sea floor through vertical rigid risers. The dry wellhead may be connected to production equipment on the platform by a “jumper hose”, or in certain applications, by telescoping devices utilized for holding the dry wellheads. In areas that experience large waves, the magnitude of resulting heave-motions caused in semi-submersibles has meant that the stroke of such telescoping devices and the length of such jumpers necessitated by the movement is too great for practical utilization with dry wellheads and rigid risers in combination with semi-submersibles.

In view of the above described deficiencies associated with the use of known designs for semi-submersible offshore platforms not specifically designed to give low heave motions, the present invention has been developed to alleviate these drawbacks and provide further benefits to the user. The aim of the present invention is to provide a configuration for a ring-pontoon styled semi-submersible platform less susceptible to heave-motion which in turn facilitates the use of steel catenary risers and/or rigid risers associated with dry wellheads. These enhancements and benefits are described in greater detail hereinbelow with respect to several alternative embodiments of the present invention.

Certain aspects of the invention are discussed in presentation papers included herewith as pages A-1 through A-6 and to be considered part of this provisional patent application filing.

DISCLOSURE OF THE INVENTION

The present invention in its several disclosed embodiments minimizes the drawbacks described above with respect to conventionally designed semi-submersible platforms, and incorporates several additional and beneficial characteristics consequent of the new designs. The present invention uniquely and advantageously combines ring-pontoon design with inclined columns to produce a semi-submersible vessel or platform having superior heave characteristics.

It has, for conventional two-pontoon semi-submersibles, been learned that the heave-motion is improved if the displacement of the submerged pontoons are moved away from the center of the semi-submersible. For two-pontoon type semi-submersibles, this has resulted in “dog-bone” shaped pontoon geometry.

The same principles instruct that a uniform configuration along the length of the elongate portions of a ring-pontoon should be modified to transfer buoyancy from the mid-length region of the elongate region of the elongate portions to the end region(s) thereof. This has been uniquely accomplished by effectively “stretching” the ring-pontoon structure in the present invention, while maintaining a substantially constant upward buoyant factor for the platform as a whole. As a result, the cross sectional area, taken vertically across the ring-pontoon, is reduced. As can be best appreciated in FIG. 4, this effectively achieves a transfer of a portion of the buoyance previously experienced mid-length of the elongate portion of the pontoon (A), and moves it to end regions thereof (B′ and B″) nearer the columns. As a result, passing waves create less heave motion in the semi-submersible platform.

Another unique feature of the present invention is the inclination of each column from corners of the ring-pontoon inwardly toward a center point of the vessel. Traditionally, inclined columns have been known, on two-pontoon type semi-submersible vessels. In the present invention, the dual-direction inclination of the columns further contributes to the semi-submersible's capabilities for resisting heave motion induced as waves pass.

The beneficial effects described above apply generally to the exemplary embodiments disclosed herein of the heave-resistant semi-submersible vessel design(s). The specific structures through which these benefits are delivered will be described in detail hereinbelow.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be described in greater detail in the following way, by example only, and with reference to the attached drawings, in which: [0015]
FIG. 1 is an elevational view of a heave-resistant semi-submersible vessel constructed according to the present invention. [0016]
FIG. 2 is a substantial cross-sectional view taken along line [0017] 2-2 of FIG. 1 showing bulkheads of the support columns configured as extensions of interior sides of the ring-pontoon.
FIG. 3 is a substantial cross-sectional view taken along line [0018] 2-2 of FIG. 1 showing bulkheads of the support columns configured as extensions of longitudinally oriented bulkheads contained within the ring-pontoon.
FIG. 4 shows the semi-submersible vessel of FIG. 3, with an illustration of a conventionally designed, non-inclined columnar system superimposed on a left-hand side thereof and in which the traditionally configured columns are substantially vertically oriented and the elongated portions of that traditional ring-pontoon have larger cross-sectional areas. [0019]
FIG. 5 is a perspective view of an exemplary embodiment of a semi-submersible vessel constructed according to the present invention. [0020]
FIG. 6 is a schematic of the box-shaped structural portion of the elongate pontoon portion demonstrating a vertical cross-section thereof.[0021]

MODE(S) FOR CARRYING OUT THE INVENTION

As required, detailed embodiments of the present invention are disclosed herein; however, it is to be understood that the disclosed embodiments are merely exemplary of the invention that may be embodied in various and alternative forms. The figures are not necessarily to scale, some features may be exaggerated or minimized to show details of particular aspects or components. Therefore, specific structural and functional details disclosed herein are not to be interpreted as limiting, but merely as a basis for the claims and as a representative basis for teaching one skilled in the art to variously employ the present invention. [0022]
With reference to the figures, a [0023] semi-submersible vessel 10 adapted to resist heave motion is shown. The semi-submersible vessel 10 includes a plurality of support columns 16, each of which has an upper end 19 and a lower end 23. A deck 39 is interconnected between the plurality of support columns 16 at locations upon the columns 16 proximate the upper ends 19 thereof. Preferably, the deck 39 is configured as a box-shaped structure. A ring-pontoon 43 is connected to the plurality of support columns 16 at locations proximate the lower ends 23 of each of the columns 16. Each support column 16 has a longitudinal axis 29 that is inclined upwardly and inwardly from the ring-pontoon 43 to the deck 39 and the longitudinal axis is oriented substantially radially from a center point 13 of the vessel 10. Though described in terms of a longitudinal axis 29 of the columns, in general, it is also the structural component of the column 16 itself that is oriented at such an inclination. While the angle of inclination may vary widely and still produce the beneficial characteristics described herein, a preferred version of the invention embodies an angle of columnar inclination of the longitudinal axis 29 of each of the support columns 16 approaching at least five degrees measured from vertical.
While it is preferred that the ring-[0024] pontoon 43 be rectangular-shaped, it is especially preferred that the ring-pontoon 43 be substantially square-shaped and have corner pontoon portions 49 interconnected by elongate pontoon portions 56. The square-shaped ring-pontoon has a centerline 73 intersecting center points 79 of vertically taken cross-sections 76 of the square-shaped pontoon 43. The elongate pontoon portions 56 constitute four box-shaped structures 59, each of which has a substantially four-sided vertically taken cross-sectional configuration.
As shown, each of the lower ends [0025] 23 of the columns 16 is connected at a corner pontoon portion 49 of the square-shaped ring-pontoon 43.
In a variation of the design, a center-[0026] point 26 of the lower end 23 of each of the columns 16 is positioned inboard 46 of the centerline 73 of the ring-pontoon 43.
A substantially triangularly-shaped [0027] tank 93 is included that has a height 96 substantially equal to a height 83 of the ring-pontoon 43. The triangularly-shaped tank 93 is positioned at a location inboard 46 of a corner pontoon portion 49 and of the lower end 23 of a column 16 connected to the ring-pontoon 43 at that corner pontoon portion 49. In a preferred embodiment, and as illustrated, there are four such triangularly-shaped tank 93, one each located inboard at the four corner pontoon portions 49.
In one embodiment, each of the [0028] support columns 16 is subdivided into four quadrants 36 by cruciform bulkheads 33. Each of the bulkheads 33 are oriented at substantially the same inclination as the longitudinal axis 29 of the support column 16. Still further, in this embodiment, each of the bulkheads 33 are arranged as extensions of interior sides 69 of the ring-pontoon 43. Alternatively, the bulkheads 33 may be arranged as extensions of longitudinally oriented bulkheads 63 contained within the ring-pontoon 43. In this instance, the longitudinally oriented bulkheads 63 divide the box-shaped portions 59 of the ring-pontoon 43 into at least two compartments 66.
In one embodiment, both an [0029] external periphery 86 and an internal periphery 89 of the ring-pontoon 43 are substantially octagonally shaped in vertical cross-section.
In another embodiment, an [0030] external surface 53 of a corner pontoon portion 49 is substantially rounded and forms an extension of a lower end 23 of a respective column 16 connected at that corner pontoon portion 49.
Regarding utilization aspects of the present invention, in one embodiment, the [0031] semi-submersible vessel 10 is adapted to accommodate its own incorporation into production processes for hydrocarbons coming from a sub-sea well-head. In one instance, the well-head is connected to the semi-submersible vessel 10 by a substantially rigid pipe laying along the sea floor and extending upwards in a catenary to the semi-submersible vessel 10. Similarly, the semi-submersible vessel 10 is also adapted for accommodating the production of hydrocarbons through a substantially vertical rigid riser terminating at a well-head positioned proximate the deck 39 and which in turn is connected to production equipment by a jumper-hose.
In an alternative embodiment, the present invention takes the form of a method for providing a semi-submersible vessel adapted to resist heave motion. The method includes the steps of providing, as described above, a plurality of [0032] support columns 16, each having an upper end 19 and a lower end 23. A deck 39 is interconnected between the plurality of support columns 16 at locations proximate the upper ends 19 thereof. A ring-pontoon 43 is connected to the support columns 16 at locations proximate the lower ends 23 of each of the columns 16. A primary characteristic of the invention includes locating a predominance of a buoyancy capacity of the ring-pontoon 43 proximate the lower ends 23 of the columns 16. The design method also includes inclining each of the plurality of support columns 16 upwardly and inwardly from the ring-pontoon 43 toward the deck 39.
The [0033] longitudinal axis 29 of each of the plurality of support columns 16, an in turn the support column 16 itself, is oriented substantially radially from a center point 13 of the semi-submersible vessel 10. As indicated above, the ring-pontoon 43 is configured into a substantially rectangular shape, and preferably a square shape.
A semi-submersible vessel adapted to resist heave motion and various of its components have been described herein. These and other variations, which will be appreciated by those skilled in the art, are within the intended scope of this invention as claimed below. As previously stated, detailed embodiments of the present invention are disclosed herein; however, it is to be understood that the disclosed embodiments are merely exemplary of the invention that may be embodied in various forms. [0034]

INDUSTRIAL APPLICABILITY

The present invention finds applicability in the offshore platform industries, and more specifically in those parts of that industry in which such offshore platforms are required to have low heave-motion characteristics. [0035]

Claims

What is claimed and desired to be secured by Letters Patent is as follows:

1. A semi-submersible vessel adapted to resist heave motion, said semi-submersible vessel comprising:

a plurality of support columns, each column of said plurality of support columns having an upper end and a lower end;

a deck interconnected between said plurality of support columns at locations proximate said upper ends of each of said columns of said plurality of support columns;

a ring-pontoon connected to said plurality of support columns at locations proximate said lower ends of each of said columns of said plurality of support columns;

each of said plurality of support columns having a longitudinal axis inclined upwardly and inwardly from said ring-pontoon to said deck.

2. The semi-submersible vessel as recited in

claim 1

wherein said longitudinal axis of each of said plurality of support columns is oriented substantially radially from a center point of said vessel.

3. The semi-submersible vessel as recited in

claim 2

wherein an angle of incline of said longitudinal axis of each of said plurality of support columns is at least five degrees from vertical.

4. The semi-submersible vessel as recited in

claim 2

wherein said ring-pontoon is substantially rectangular-shaped.

5. The semi-submersible vessel as recited in

claim 2

wherein said ring-pontoon is substantially square-shaped having corner pontoon portions connected by elongate pontoon portions, and said square-shaped pontoon having a centerline intersecting center points of vertically taken cross-sections of said square-shaped pontoon.

6. The semi-submersible vessel as recited in

claim 5

wherein each of said lower ends of said columns of said plurality of columns is substantially connected at a corner pontoon portion of said square-shaped ring-pontoon.

7. The semi-submersible vessel as recited in

claim 6

wherein said deck is positioned substantially inboard of said centerline of said ring-pontoon.

8. The semi-submersible vessel as recited in

claim 6

wherein said elongate pontoon portions comprise four box-shaped structures, each of said box-shaped structures having a substantially four-sided vertically taken cross-sectional configuration.

9. The semi-submersible vessel as recited in

claim 6

wherein a center-point of said lower end of each of said columns of said plurality of columns is positioned inboard of said centerline of said ring-pontoon.

10. The semi-submersible vessel as recited in

claim 9

, further comprising:

a substantially triangularly-shaped tank having a height substantially equal to a height of said ring-pontoon, said substantially triangularly-shaped tank positioned at a location inboard of a corner pontoon portion and of a said lower end of a column connected to said ring-pontoon at said corner pontoon portion.

11. The semi-submersible vessel as recited in

claim 9

, further comprising:

each of said support columns being subdivided into four quadrants by cruciform bulkheads, each of said bulkheads being oriented at substantially the same inclination as said longitudinal axis of said support column; and

each of said bulkheads being arranged as extensions of inward sides of said ring-pontoon.

12. The semi-submersible vessel as recited in

claim 9

, further comprising:

each of said support columns being subdivided into four quadrants by cruciform bulkheads, each of said bulkheads being oriented at substantially the same inclination as said longitudinal axis of said support column;

each of said bulkheads being arranged as extensions of longitudinally oriented bulkheads contained within said ring-pontoon; and

said longitudinally oriented bulkheads dividing said pontoon box-shaped structures into at least two compartments.

13. The semi-submersible vessel as recited in

claim 9

, further comprising:

an external periphery of said ring-pontoon being substantially octagonally shaped.

14. The semi-submersible vessel as recited in

claim 9

, further comprising:

an internal periphery of said ring-pontoon being substantially octagonally shaped.

15. The semi-submersible vessel as recited in

claim 9

, further comprising:

an external surface of a corner pontoon portion being substantially rounded and forming an extension of a lower end of a respective column connected at said corner pontoon portion.

16. The semi-submersible vessel as recited in

claim 9

, wherein said deck is configured as a box-shaped structure.

17. The semi-submersible vessel as recited in

claim 9

, wherein said semi-submersible vessel is adapted for accommodating the production of hydrocarbons from at least one sub-sea well-head that is connected to said semi-submersible vessel by at least one substantially rigid pipe laying along the sea floor and extending upwards in a catenary to said semi-submersible vessel.

18. The semi-submersible vessel as recited in

claim 9

, wherein said semi-submersible vessel is adapted for accommodating the production of hydrocarbons through a substantially vertical rigid riser terminating at a well-head positioned proximate said deck and which in turn is connected to production equipment by a jumper-hose.

19. A method for providing a semi-submersible vessel adapted to resist heave motion, said method comprising:

providing:

a deck interconnected between said plurality of support columns at locations proximate said upper ends of each of said columns of said plurality of support columns; and

locating a predominance of a buoyancy capacity of said ring-pontoon proximate said lower ends of said columns of said plurality of support columns; and

inclining each of said plurality of support columns upwardly and inwardly from said ring-pontoon to said deck.

20. The method as recited in

claim 19

, further comprising:

orienting a longitudinal axis of each of said plurality of support columns substantially radially from a center point of said vessel.

21. The method as recited in

claim 19

, further comprising:

configuring said ring-pontoon into a substantially rectangular shape.

22. A semi-submersible vessel adapted to resist heave motion, said semi-submersible vessel comprising:

at least four of said plurality of support columns having a longitudinal axis inclined upwardly and inwardly from said ring-pontoon to said deck and said longitudinal axis being oriented substantially radially from a center point of said vessel.

23. A semi-submersible comprising:

a superstructure deck;

a ring pontoon having four box-structures having a substantially four-sided cross-section and formed into a symmetrical square ring;

four columns inclined inwards toward the upper end with an angle of inclination of at least five degrees from vertical; and

a longitudinal axis of said pontoon box-structures being located outside of lower end center-points of said columns.